Electron-optical corrector for eliminating third-order aberations

ABSTRACT

The invention relates to an electron-optical corrector for eliminating third-order aberrations, such as spherical aberrations, field curvature and off-axis astigmatism; said corrector being devoid of third-order off-axis coma, third-order distortion and first-order chromatic aberration of the first degree. The corrector has a construction which is symmetrical about the central plane in the direction of the linear optical axis. A hexapole S 1  of length l 1  is first positioned in the direction of the beam path, followed by a circular lens R 1 , a hexapole S 2  of length l 2  and subsequently a circular lens R 2  which is followed by a third hexapole S 3  with the same strength with the same strength of the hexapole S 1  and double the length of the latter l 3 =21 1 . The separation of the two circular lenses R 1 , R 2  and the distance from the circular lens to the first hexapole S 1  is chosen in such a way that the internal plane of S 1  comes to rest in the front principal focus of the circular lens that is positioned downstream and the center of the hexapoles S 2  and S 3  is located on the focal plane. Additional elements of the corrector also follow in sequence, said elements being symmetrical about the central plane Z m  of the hexapole S 3 .

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/DE01/00102 which has an Internationalfiling date of Jan. 12, 2001, which designated the United States ofAmerica.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electron-optical corrector for eliminatingthird-order aberrations, such as spherical aberrations, field curvatureand off-axis astigmatism. The corrector being devoid of third-orderoff-axis coma, third-order distortion and first-order chromaticaberrations of the first degree. The corrector has a construction whichis symmetrical about the central plane in the direction of the linearoptical axis. A hexapole S₁ of length l₁ is first positioned in thedirection of the beam path, followed by a circular lens R₁, a hexapoleS₂ of length l₂.

2. Description of the Background Art

The efficiency of electron-optical systems, which in the sense of thisinvention are also understood to include those with ion-imaging systems,is limited by their image aberrations, of which, depending on thespecific application and the extent of the corrections already made,particular image aberrations are responsible for limiting theperformance, the elimination of which represents considerable progressin the improvement of electron-optical systems. It is possible tosystematically subdivide and classify the image aberrations into axialimage aberrations, which are also determined by the fundamental pathsemerging in the two sections of the optical axis in the object plane,off-axis image aberrations, which in turn are dependent on thefundamental paths emerging outside the optical axis in the object plane,and chromatic aberrations, which only occur with different speeds of theimaging electrons. With magnifying electron-optical systems, such asthose used in electron microscopy, it is most important to eliminate theaxial image aberrations to increase efficiency. With reducingelectron-optical systems, such as those used in lithography forinscribing objects by means of electron beams, the elimination ofoff-axis image aberrations is decisive.

The aim is always to set up and adjust, in its entirety, the systemcomprising the imaging lens system and the corrector such that theefficiency-limiting image aberrations of the entire system areeliminated or substantially minimised, the corrector having the functionof, on one hand, achieving, by negative image aberration coefficients,an elimination or at least a reduction, and on the other hand notcausing an increase, of disadvantageous image aberration coefficients.

Optik 69 No. 1 (1984) 24-29 discloses the use of two hexapoles for thecorrection of electron-optical image aberrations in circular lenses.These correction elements are also used for eliminating the first orderspherical aberration (Optik 60 No. 3 (1982) 271-281).

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide anelectron-optical corrector, with the aid of which, in addition to thefirst order, first degree chromatic aberration, all third-order imageaberrations, such as spherical aberrations, field curvature, off-axisastigmatism, off-axis coma, and distortion are eliminated in such a waythat no additional off-axis image aberration is generated.

This object is achieved according to the invention in that it isfollowed by a circular lens (R₂), which is followed by a third hexapole(S₃) with the same strength as the hexapole (S₁) and twice the length13=211 thereof, the separation of the two circular lenses (R₁, R₂) ofthe same strength being 2f of the focal length, the hexapole (S₂) beingpositioned in the principal focus between the two circular lenses (R₁,R₂), and the distance of the circular lens from the first hexapole (S₁)being chosen such that the plane of the (S₁), which is assigned to thecorrector centre, comes to lie in the front principal focus of thecircular lens that is positioned downstream, and the centre of thehexapoles (S₂) and (S₃) is positioned in the focal plane of the circularlenses (R₁, R₂) and this is finally followed by two further circularlenses (R₃, R₄) and hexapoles (S₄, S₅), which are set up symmetricallyto the centre plane Z_(m) of the hexapole (S₃).

To produce the required telescopic Gaussian ray path, the distancebetween adjacent circular lenses is in each case 2f, f representing thefocal length of the circular lens. The hexapoles S₂, S₄ and S₃ are to bepositioned with their centres, in each case, being positioned in theprincipal focus of the circular lenses. The hexapoles S₁ and S₅ areassigned such that their respective inner plane comes to lie in thefocal point of the circular lens. To prevent the occurrence of secondorder aberrations, the strength of the hexapole lenses S₁, S₃ and S₅ isin each case the same. In addition, S₂=S₄ is adjusted, however theirstrength can be chosen completely independently of S₁, S₃ and S₅.However, a symmetry with respect to the central plane Z_(m) in the setup and adjustment must always be ensured.

To eliminate spherical aberrations, field curvature and off-axisastigmatism (all third-order aberrations), the intensity of thehexapoles S₁ and S₂ are available and parameters that can be chosenfreely and independently of one another. By corresponding setting, twoof the aforementioned image errors in the entire system can beeliminated. For purposes of lithography, it is most important toeliminate off-axis astigmatism as well as the field curvature. Althoughthe third-order spherical aberration of the corrector is negative andleads to a reduction of the overall aberration, however, in the mostgeneral cases it does not become zero. By means of an appropriatespatial positioning in the overall system and corresponding choice ofdistance from the adjacent lenses, it can be achieved for a particularenlargement that the third-order spherical aberration of the entiresystem can be compensated.

In addition to the elimination of the aforementioned image aberrations,the corrector itself is also free of third-order distortion and coma andof first-order, first degree off-axis chromatic aberrations, so thatwhen the corrector is installed in a correspondingly aberration-freeelectron-optical system the entire system also remains free of theseaberrations. For a magnification unequal to 1, theoreticalconsiderations require that a system free of off-axis coma anddistortion must consist of at least four lenses.

The decisive advantages of the proposed corrector include, withappropriately setting and spatial positioning of the sphericalaberrations, adjusting the field curvature and the off-axis astigmatism(all third order) such that the entire system becomes aberration free,but also that further image aberrations, namely third order distortion,off-axis coma and first order, first degree chromatic aberration of theentire system are not increased by virtue of the corrector, since it isitself aberration-free.

In the most general case, the hexapoles are always aligned relative toone another in the same section. In cases where magnetic lenses areused, the magnetic field leads to an anisotrope (azimuthal component),which leads to image rotation. For aberration elimination, a rotation ofthe sections of the hexapoles S₂ and S₄ which lie in the same section,occurs relative to the hexapoles S₁, S₃ and S₅, which are also alignedin a common section. The angle of rotation is determined by themagnitude of the anisotropic component determined by the magnetic field.

The rotation of the hexapoles can then take place with maintenance ofthe mechanical orientation by electrostatic means, if, to generate thehexapole field, a twelve-pole is used, which permits a rotation of thesections about any desired angle by corresponding repoling of theelectrodes. It correspondingly becomes possible, after installation ofthe twelve poles, to rotate the hexapole field by the desired angle bycorresponding change of the poling.

The corrector proposed above eliminates image aberrations up to thirdorder. In a particularly preferred embodiment, two of these correctivesare positioned one behind the others in series and optically connectedto one another by a circular lens doublet. The distance of the twocircular lenses 2f and the distance of the last hexapole of the firstcorrector to the first circular lens, and the distance between thesecond circular lens and the adjacent hexapole is also equal to thefocal length f. By means of such a linking, an antisymmmetric beam pathis obtained in both correctors, which leads to mutual compensation ofthe fourth-order aberrations of the two individual correctives. Sincethe remaining circular lenses are free of fourth order aberrations, onlyfifth and higher order aberrations remain. A corrector with this opticalproperty is designated as a double astigmat.

The corrector proposed according to the invention is preferably used ina size-reducing electron-optical system, as is used in lithography, andwhich is used to reduce in size particular structures defined by a maskby the electron-optical system and impress and write them on a crystal(wafer) located in the image plane by the incident electrodes. Theessential factor is the creation of electronic devices and integratedcircuits with the smallest possible dimensions, the electron-opticalsystems, in comparison to light-optical images, having the advantage ofbeing able to reproduce much finer structures because of their muchsmaller wavelength. Here, the corrector is brought into the beam pathdownstream of the projection lens located behind the object plane and,at the output side—if appropriate with the interposition of a transferlens—the operating part (wafer) is written on via an objective in theimage plane. The term objective lens can be interpreted broadly, and mayinclude a system composed of a plurality of lenses. The proposedcorrector has the particular advantage to be able to eliminate imageaberrations limiting the effectiveness of electron-optical size-reducingimaging systems. A considerable improvement of the imaging quality isconsequently to be expected.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus, are not limitiveof the present invention, and wherein:

The FIGURE shows a section along the optical axis and reproduction ofthe Gaussian fundamental paths.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The diagrammatic illustration is kept in a schematic view. The hexapolesare here represented by S_(i), their lengths by l_(i) and the circularlenses by R_(i). The corrector has the following construction:

-   -   At the inlet side, there is located the first hexapole S₁ of        length L₁. This is followed by the circular lens doublet having        the circular lenses R₁ and R₂. The focal length of the two        circular lenses R₁ and R₂ being identical, and their distance        being chosen as equal to 2f, f being the focal length, such that        the circular lens doublet in its totality generates a telescopic        beam path. Between the two circular lenses R₁, R₂, the second        hexapole S₂, of length l₁, is located in the focal plane. The        first hexapole S₁ is located at a distance such that its inner        plane comes to lie in the focal point of the downstream circular        lens. The third hexapole S₃ in the direction of the beam path is        chosen such that in its intensity it is equal to the first        hexapole S₁, but of double its length l₃=21₁. The spatial        assignment takes place such that the centre plane of the third        hexapole S₃ comes to lie in the focal plane Z_(n) of the        circular lens.

The further construction of the corrector in the direction of the beampath is symmetrical to this central plane Z_(m), so that both as regardsthe spatial positioning as well as the chosen pole strength, referencecan be made to the previous models in order to avoid repetitions.

The above-described corrector has two freely selectable parameters,namely the strength of the hexapole S₁ (and thereby also that of thehexapoles S₃ and S₅) as well as the strength of the hexapole S₂ (andthereby also that of the hexapole S₄). These two parameters permit twoof the following three image aberrations to be freely set, namelythird-order field curvature, off-axis astigmatism and sphericalaberrations, in such a manner that compensation of two of these imageaberrations occurs. As already described, the third aberration(described by means of the third order spherical aberration) can beinfluenced in the desired way by appropriate choice of the geometricalparameters, and in particular the distance, so that extensivecompensation of the third image aberration is possible.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are to beincluded within the scope of the following claims.

1. An electron-optical corrector for eliminating third orderaberrations, such as spherical aberrations, field curvature and off-axisastigmatism; said corrector being devoid of third-order off-axis coma,third-order distortion and first-order chromatic aberrations of thefirst degree, the corrector having a construction which is symmetricalabout the central plane in the direction of the linear optical axis,wherein a first hexapole of a first length is first positioned in thedirection of the beam path, followed by a first circular lens, a secondhexapole of a second length, followed by a second circular lens, whichis followed by a third hexapole with the same strength as the firsthexapole and twice the first length of the first hexapole, theseparation of the first circular lenses of the same strength being 2f ofthe focal length, the second hexapole being positioned in the principalfocus between the first and second circular lenses, and the distance ofthe circular lens from the first hexapole being chosen such that theinner plane of the first hexapole comes to lie in the front principalfocus of the circular lens that is positioned downstream, and the centerof the second and third hexapoles is positioned in the focal plane andthis is finally followed by further elements of the corrector, which areset up symmetrically to the center plane of the third hexapole.
 2. Theelectron-optical corrector according to claim 1, wherein the first andsecond hexapole strengths are in each case chosen so as to eliminate theoff-axis astigmatism, as well as the field curvature of the entiresystem, and the distance from the adjacent lenses of the entire systemis chosen so that the third-order spherical aberration can be influencedto the extent of compensation.
 3. The electron-optical correctoraccording to claim 1, wherein the section of the first and a fourthhexapole is azimuthally rotated about the optical axis with respect tothe section formed by the first, third and a fifth hexapole.
 4. Theelectron-optical corrector according to claim 3, wherein the first,second, third, fourth and fifth hexapoles are generated in a twelve-poleelement comprising 12 electrodes or pole shoes.
 5. The electron-opticalcorrector according to claim 1, wherein two of the aforementionedcorrectors are positioned in series by means of a circular lens doublet,the distance between the two circular lenses corresponding to twicetheir focal length, and the distance of a last hexapole of the firstcorrector to the first circular lens, and the same applying to thedistance of the second circular lens of the circular lens doublet to thefirst hexapole.
 6. Use of an electron-optical corrector according toclaim 1 in a size-reducing electron-optical system, in particularlithography, wherein the mask to be imaged is located in the objectplane and a wafer is located in the image plane, comprising a projectionlens located downstream of the object plane in the direction of the beampath and an objective lens located upstream of the image plane, whereinthe corrector is inserted in the beam path downstream of the projectionlens.
 7. Use of an electron-optical corrector according to claim 1 in asize-reducing electron-optical system, in particular lithography,wherein the mask to be imaged is located in the object plane and a waferis located in the image plane, comprising a projection lens locateddownstream of the object plane in the direction of the beam path and anobjective lens located upstream of the image plane, wherein a transferlens between the corrector and objective lens.
 8. An electron opticalcorrector comprising: a first hexapole; a first circular lens; a secondhexapole; a second circular lens; and a third hexapole, wherein saidfirst hexapole is arranged in front of said first circular lens, whereinsaid second hexapole is arranged between said first circular lens andsaid second circular lens, and wherein said third hexapole is arrangeddownstream from said second circular lens.
 9. The electron opticalcorrector of claim 8, wherein said first hexapole has a first length,said third hexapole has a second length, and wherein said second lengthis twice said first length.
 10. The electron optical corrector of claim8, wherein said first circular lens and said second circular lens havean identical focal length, and wherein between said first circular lensand said second circular lens a distance is provided which is twice saidfocal length.
 11. The electron optical corrector of claim 10, whereinsaid first circular lens and said second circular lens define atelescopic system with an inlet focal plane upstream of said firstcircular lens and a principal focal plane between said first and saidsecond circular lens, and wherein said first hexapole is positioned suchthat an inner plane of said first hexapole lies in said inlet focalplane and said second hexapole being positioned in said principal focalplane.
 12. The electron optical corrector of claim 8, wherein said firsthexapole has a first strength, said second hexapole has a secondstrength, wherein said first and second strengths are determined so asto eliminate an off axis astigmatism as well as a field curvature of anentire optical system, and wherein a distance of said corrector fromadjacent lenses of said entire system is determined so that athird-order spherical aberration can be influenced.
 13. The electronoptical corrector of claim 8, wherein said first, second and thirdhexapoles each have sections and wherein said section of one of saidfirst, second, and third hexapole is azimuthally rotated with respect tosaid sections of other sections of said first, second, and thirdhexapoles.
 14. The electron optical corrector of claim 8, wherein saidat least one of said first, second, and third hexapoles are generated ina twelve pole element having twelve electrodes or twelve pole shoes. 15.An electron optical corrector comprising: a first hexapole; a firstcircular lens; a second hexapole; a second circular lens; a thirdhexapole; a third circular lens; a fourth hexapole; a fourth circularlens; and a fifth hexapole, wherein said first hexapole is arranged infront of said first circular lens, wherein said second hexapole isarranged between said first circular lens and said second circular lens,wherein said third hexapole is arranged between said second and saidthird circular lens, wherein said fourth hexapole is arranged betweensaid third circular lens and said fourth circular lens, wherein saidfifth hexapole is arranged behind said fourth circular lens, and whereinsaid electron optical corrector is symmetrical to a center plane of saidthird hexapole.
 16. The electron optical corrector of claim 15, whereinsaid first hexapole has a first length, said third hexapole has a secondlength, and wherein said second length is twice said first length. 17.The electron optical corrector of claim 15, wherein said first circularlens and said second circular lens have an identical focal length, andwherein between said first circular lens and said second circular lens adistance is provided which is twice said focal length.
 18. The electronoptical corrector of claim 17, wherein said first circular lens and saidsecond circular lens define a telescopic system with an inlet focalplane upstream of said first circular lens and a principal focal planebetween said first and said second circular lens, and wherein said firsthexapole is positioned such that an inner plane of said first hexapolelies in said inlet focal plane and said second hexapole being positionedin said principal focal plane.
 19. The electron optical corrector ofclaim 15, wherein said first hexapole has a first strength, said secondhexapole has a second strength, wherein said first and second strengthsare determined so as to eliminate an off-axis astigmatism as well as afield curvature of an entire optical system, and wherein a distance ofsaid corrector from adjacent lenses of said entire system is determinedso that a third-order spherical aberration can be influenced.
 20. Theelectron optical corrector of claim 15, wherein said first, second, andthird hexapoles each have sections, and wherein said section of one ofsaid first, second, and third hexapole is azimuthally rotated withrespect to said sections of other sections of said first, second, andthird hexapoles.
 21. The electron optical corrector of claim 15, whereinsaid at least one of said first, second, and third hexapoles aregenerated in a twelve pole element having twelve electrodes or twelvepole shoes.
 22. An electron optical system comprising: an object plane;a projection lens; and an art image plane; wherein said projection lensimages said object plane into said image plane and an electron opticalcorrector, said electron optical corrector comprising: a first hexapole;a first circular lens; a second hexapole; a second circular lens; athird hexapole; a third circular lens; a fourth hexapole; a fourthcircular lens; and a fifth hexapole, wherein said first hexapole isarranged in front of said first circular lens, wherein said secondhexapole is arranged between said first circular lens and said secondcircular lens, wherein said third hexapole is arrange between saidsecond and said third circular lens, wherein said fourth hexapole isarranged between said third circular lens and said fourth circular lens,and wherein said fifth hexapole is arranged downstream from said fourthcircular lens, and wherein said electron optical corrector issymmetrical to a center plane of said third hexapole.
 23. The electronoptical system of claim 22, wherein said electron optical corrector isarranged downstream of said projection lens.
 24. The electron opticalsystem of claim 22, further comprising an objective lens upstream ofsaid object plane.
 25. The electron optical system of claim 22, whereinsaid corrector is arranged upstream of said object plane and wherein atransfer lens is provided between said objective lens and said electronoptical corrector.
 26. An electron optical corrector comprising: a firstcorrector component and a second corrector component, which are ofidentical design and arranged in series; a circular lens doubletarranged between said first and second corrector component, said firstand second corrector component each comprising a first hexapole; a firstcircular lens; a second hexapole; a second circular lens; a thirdhexapole; a third circular lens; a fourth hexapole; a fourth circularlens; and a fifth hexapole, wherein said first hexapole is arranged infront of said first circular lens, wherein said second hexapole isarranged between said first circular lens and said second circular lens,wherein said third hexapole is arranged between said second and saidthird circular lens, wherein said fourth hexapole is arranged betweensaid third circular lens and said fourth circular lens, wherein saidfifth hexapole is arranged downstream from said fourth lens, and whereinsaid electron optical corrector is symmetrical to a center plane of saidthird hexapole.